The Nervous System

The nervous system is made up of a complex collection of nerves and specialised cells which allow us to response to our environment. Whenever two nerve cells (neurones) meet, they form a synapse. Our brains form so many synapses that they outnumber the number of stars in the galaxy. It’s this that allows us to carry out complex behaviours and be just so bloody brilliant..

How the nervous system works

The nervous system detects changes in our environment (known as stimuli) through cells called receptors. Receptors are sensitive to a number of different aspects of our environment, such as light, pressure (touch) and chemicals in the air (smell). When receptors detect certain stimuli, they signal to the central nervous system (CNS) through initiating an electrical impulses through a neuron (nerve cell). The neuron which sends an electrical impulse from the receptor within the sense organ and the coordination centre is called the sensory neuron. The coordination centre receives impulses from various receptors around the body, processes the information and coordinates a response by signalling to other parts of the body. Coordination centres include the brain, spinal cord and pancreas. These organs will signal to an effector (a muscle or gland) by releasing an electrical impulse along a motor neuron. Stimulation of an effector will produce a response such as muscle contraction or hormonal release.

Receptors

Our nervous system uses receptors to detect stimuli (changes in the environment) and pass on this information to the CNS. Receptors can either be whole cells (e.g. photoreceptors are cells which are sensitive to light) or proteins molecules which are found on the cell surface membrane. Each receptor is specific to a single type of stimulus, such as light, temperature or glucose concentration. When a receptor is not stimulated, there is a charge difference between the inside and outside of the membrane and it is said to be polarised. When the receptor detects a stimulus, the permeability of its cell membrane changes which changes the charge difference (potential difference) across the membrane. If the change in potential difference is large enough (i.e. it exceeds the threshold level), it will trigger an action potential (an electrical impulse) in a sensory neuron.

We contain the following receptors in our sense organs:

• Chemoreceptors - receptors which detect chemicals

• Thermoreceptors - receptors which detect heat

• Mechanoreceptors - receptors which detect pressure (see the Pacinian corpuscle below)

• Photoreceptors - receptors which detect light (e.g. rods and cones)

Photoreceptors

Photoreceptors are receptors which detect light and are found in the retina of the eye. There is an area of the retina called the fovea which contains a cluster of photoreceptor cells. Photoreceptors detect light as it hits the retina and send nerve impulses to the brain along the optic nerve. The region of the eye containing the optic nerve is called the blind spot since there are no photoreceptors in this region so light can’t be detected. Photoreceptors are connected to the optic nerve through a bipolar neurone.

The human eye has two types of photoreceptors - rods and cones. Rods are mostly located along the outside of the retina while cones are clustered together in the fovea. Rods are responsible for black-and-white vision and can function in lower light levels than cones. They are much more sensitive than cones, so are the type of photoreceptor used for visualising objects in the dark. Cones are responsible for colour vision and are sensitive to either blue, green or red light. Different cones are stimulated in different proportions, so that we see different colours. Cone cells provide good visual acuity (the ability to distinguish between two points which are close together) because each cone cell has its own synapse via a bipolar neurone which connects to the optic nerve.

In dark conditions, the membrane of rod cells is depolarised, which means there is not much difference in charge between the inside and outside of the membrane. This is because the rod cells actively transport sodium ions out of the cell, which flow straight back into the cell through sodium ion channels. Depolarisation of the rod cell membrane triggers the release of neurotransmitters which inhibit the bipolar neurone. The bipolar neurone cannot fire an action potential which means that no information is sent to the brain. However, when light is present, light energy causes a pigment called rhodopsin to split apart into two proteins, retinal and opsin. This process is referred to as the bleaching of rhodopsin and it causes sodium ion channels in the cell surface membrane to close. Sodium ions continue to be actively transported out of the rod cells but they cannot flow back into the cell through the ion channels, creating a difference in charge across the membrane. The inside of the membrane is now much more negative compared to the outside and the rod cell is said to be hyperpolarised. When the rod cell is hyperpolarised it stops releasing neurotransmitters. This means that inhibition of the bipolar neurone stops and it can now become depolarised. If depolarisation exceeds a threshold level, information is passed onto the brain via the optic nerve and the presence of light is detected.

Pacinian corpuscles

Pacinian corpuscles are receptors which respond to changes in pressure - they are a type of mechanoreceptor. They are found deep in the skin and are abundant in the feet, fingers, external genitalia and in our joints. The Pacinian corpuscle consists of a single sensory neurone, surrounded by layers of tissue which are each separated by a gel, forming an onion-like structure.

The Pacinian corpuscle contains stretch-mediated sodium ion channels in the cell surface membrane. Under normal conditions these channels are closed but when pressure is applied these channels become deformed and open, allowing a rapid influx of sodium ions. This makes the membrane potential in the neurone less negative (depolarisation), producing a generator potential which can then produce an action potential.

Types of neurone

Neurones are cells which carry information to and from the central nervous system, in the form of electrical impulses called action potentials. There are three different types of neurone, with slightly different structures. What they all have in common, however, is a cell body containing a nucleus, dendrites which carry an action potential towards the cell body and an axon which carries the action potential away from the cell body.

• Sensory neurones carry action potentials from receptors to the central nervous system. They consist of one long dendron and a short axon.

• Relay neurons carry action potentials between the sensory and motor neurons and are found within the CNS. They have lots of short dendrites.

• Motor neurones carry action potentials from the CNS to an effector. They have lots of short dendrites and one long axon.

Resting potential

When a neurone is not firing (i.e. it is not transmitting an action potential), there is a difference in charge between the inside and the outside of the membrane (it is polarised). This charge difference is referred to as the resting potential and is usually around -70 mV. Polarisation of neuronal cell membranes at rest occurs due to the action of sodium-potassium ion pumps. These pumps are found within the cell membrane and actively transport sodium and potassium ions into and out of the neurone. For every three sodium ions that the proteins pump out of the cell, they pump two potassium ions into the cell. This ensures that there are always more positive ions out of the cell compared to inside the cell and makes sure there is a charge difference across the membrane.

Action potential

When a neurone is stimulated, the charge difference between the inside and outside of the cell membrane is lost and the membrane is depolarised. If enough charge is lost and depolarisation exceeds -55 mV, an action potential will occur in that neurone. The -55 mV ‘limit’ is known as the threshold potential - any depolarisation above this number will result in an action potential whereas anything less than that will result in nothing. We therefore refer to action potentials as an “all-or-nothing” response.

Depolarisation during an action potential occurs because sodium ion channels open up in the membrane. Remember that the sodium-potassium ion pump has been actively transporting sodium ions out of the neurone, creating a sodium ion concentration gradient. This means that when sodium ion channels open, sodium ions flood into the neuron by facilitated diffusion. The potential difference across the membrane is reduced until is reaches a voltage of around +30 mV.

Sodium ion channels close and potassium ion channels open, which causes potassium ions to move out of the neurone down their concentration gradient. The movement of positive ions out of the cell means that there is a charge difference again across the membrane - this is called repolarisation. However, the charge difference exceeds the resting potential and becomes ‘hyperpolarised’. This is because the potassium ion channels are slow to close and too many potassium ions diffuse out of the neurone. The action of the sodium-potassium ion pump restores the balance between sodium and potassium ions on either side of the membrane and returns the neurone to its resting potential of -70 mV.

Immediately after an action potential is a brief period called the refractory period. During the refractory period, the neurone cannot be stimulated and an action potential cannot occur. This is because the ion channels are recovering and they cannot be made to open. The refractory period is important because it ensures that action potentials do not overlap (i.e. they pass along the neurone as separate impulses) and that action potentials are unidirectional.

Once an action potential occurs in one part of the neuron, it will stimulate an action potential in the adjacent part of the neuron, creating a kind of ‘Mexican wave’ of depolarisation. This wave of depolarisation occurs because the sodium ions which diffuse into the neuron diffuse sideways, causing voltage-gated ion channels in the next portion of the neurone to open, so sodium ions move into the neurone further along the membrane. The wave moves away from the part of the neurone which has just fired an action potential because that part of the neurone will be in the refractory period and cannot be stimulated.

Saltatory conduction

Some neurones are insulated with a fatty layer along the axon. This fatty layer is called a myelin sheath and is made up of a type of cell called a Schwann cell. The myelin sheath acts as an electrical insulator, which means that ions cannot move into or out of the myelinated portions of the neurone. However, there are gaps in the myelin sheath called nodes of Ranvier, where sodium ion channels and potassium ion channels are concentrated. Action potentials occur only at the nodes of Ranvier - when one node is stimulated this triggers depolarisation of the next node, causing the impulse to ‘jump’ from node to node. This type of nervous transmission is called saltatory conduction and is much faster than transmission along non-myelinated neurones, where the action potential has to travel along the entire length of the neurone in a wave of depolarisation. The speed at which an action potential moves along a neurone is known as the conduction velocity - the higher the conduction velocity, the faster the action potential is travelling. This means that action potentials along myelinated neurones have a higher conduction velocity compared to those travelling along non-myelinated neurones.

Size of the stimulus

We’ve seen how an action potential is an ‘all-or-nothing’ response. If the threshold potential is reached, an action potential will occur. This action potential is always of the same voltage (depolarisation to +30 mV) regardless of whether the stimulus that initiated the action potential is small (e.g. a pinprick) or large (e.g. a sledgehammer). If the threshold isn’t reached then an action potential will not be fired. The difference between action potentials resulting from stimuli of different sizes is the frequency that action potentials are firing - the bigger the stimulus, the more often an action potential will occur along the neurone.

Anaesthetics

Anaesthetics are drugs which create a numbing sensation and are used in medicine to prevent patients from feeling pain (e.g. during an operation). They work by binding to sodium ion channels in the neurone and preventing them from opening. If sodium ions cannot move into the neurone, then the membrane cannot depolarise and an action potential cannot occur. This prevents neurone from sending a pain impulse to the brain, so the brain doesn’t register anything.

Synapses

A synapse is a gap found between neurones (or between a motor neurone and an effector). Electrical impulses cannot pass through the gap, so neurones release neurotransmitters from one neurone to the next to stimulate an action potential in the next neurone. The neurone before the synapse is called the presynaptic neurone and the one after the synapse is called the postsynaptic neurone. The space between them is called the synaptic cleft. This means that action potentials will travel along the presynaptic neurone, through the synaptic cleft (via neurotransmitters) then along the postsynaptic neurone. The presynaptic neurone has a swelling at the end which is called the synaptic knob.

Synaptic transmission takes place in the following stages:

1. An action potential arrives at the end of the presynaptic neurone (at the synaptic knob) and triggers the opening of voltage-gated calcium ion channels.

2. Calcium ions move into the synaptic knob by facilitated diffusion and trigger the movement of vesicles containing neurotransmitters (such as acetylcholine or dopamine) towards the presynaptic membrane.

3. The vesicles fuse with the presynaptic membrane and their contents is released by exocytosis.

4. The neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic membrane.

5. This triggers the opening of sodium ion channels in the postsynaptic membrane. Sodium ions move into the postsynaptic neurone, causing depolarisation and triggering an action potential if the excitation exceeds the threshold potential of -55 mV.

6. The neurotransmitter is removed from the synaptic cleft which prevents the continuous stimulation of an action potential in the postsynaptic neurone. The neurotransmitter is either reabsorbed by the presynaptic neurone (and recycled) or broken down by enzymes in the synaptic cleft (and the products are reabsorbed).

Receptors which are complementary to neurotransmitters, such as acetylcholine, are only found on the postsynaptic membrane - there are none within the postsynaptic membrane). This ensures that the action potential always travels in one direction only (unidirectional) and prevents the nerve impulse from travelling backwards.

Synaptic divergence and convergence

When one neurone forms connections to multiple neurones through mutliple synapses, the action potential can diverge, sending information to different parts of the body. This is known as synaptic divergence. On the other hand, multiple neurones can all connect to a single neurone, causing the action potentials from multiple neurones to converge and become amplified (synaptic convergence). This creates a stronger impulse when many different neurones are activated that form part of the same neural pathway.

Habituation

Habituation is a kind of learned response where organisms learn to ignore unimportant stimuli (i.e. they know that they are not dangerous or do not offer any reward) after repeated exposure. It’s what happens if you live close to a hospital and start to blank out the sound of sirens, for example. It’s important because it means that organisms don’t waste time and energy by responding to stimuli that do not pose a danger to them.

When habituation occurs, the action potentials that result from the stimulus dampen down over time. The repeated exposure to the stimulus decreases the amount of calcium ions which enter the presynaptic neurone, which means that less vesicles containing neurotransmitters release their contents into the synaptic cleft. This means that there are less neurotransmitters to bind to receptors on the postsynaptic neurone, so less sodium ions channels open in the postsynaptic neurone. Less depolarisation of the membrane occurs, which may not reach the threshold potential. This means that fewer action potentials will reach the effector (the muscle or the gland) which carries out the response.

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